ath9k_hw: fix division by zero in the ANI monitor code

The commit "ath9k_hw: remove code duplication in phy error counter handling"
split off some duplicate code into a separate function, but did not have a
return code for aborting ANI processing based on counter values.
This introduced a divide by zero issue.
This patch adds the missing return code check in ath9k_hw_ani_monitor

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>